1. Field of the Invention
This invention pertains to the structure of rotary cone rock bits. More specifically, this invention relates to the manner in which rock cutting cones are rotatively supported, located, and oriented with respect to a pre-determined and controlled rock-cutting geometry.
2. Description of the Prior Art
This discussion is limited to rock bits having a plurality of rotating toothed cutters which are generally conical in form. The conical rock cutters are rotatively borne upon downwardly and inwardly directed cantilevered journal shafts which depend from a structural body. The upper portion of the body is threaded for attachment to the lower end of a drill line made of pipe. The bit body also serves the function of a terminal pipe fitting to control and route a fluid flow from the drill line pipe to exit through the plurality of fluid nozzles housed therein.
In use, the drill line pipe is rotated while forcing the rock bit into the earth. The rock cutter cones, with their vertices directed toward the vertical centerline of the drill bit, roll about the vertical centerline of the drill bit as the rock cutting teeth are forced into the geologic formation to crush and fracture rock. Fluid pumped down the drill line and through the nozzles serves both to dissipate the heat of drilling, and to flush rock cuttings from the drilling zone and upward to the earths surface through the annular space between the bore hole wall and the drill line pipe.
To permit assembly of the rock cutter cones upon their respective journal shafts, the structural body of the rock bit is conventionally made in separate longitudinal segments, called "Legs", each leg incorporating one journal shaft. The segments are welded into an integral unit after being assembled with the cutters. After welding, the body is threaded for attachment to the lower end of the drill line.
Inventors in the art have long recognized the advantages in production of a rotary rock bit designed with a "one-piece" body structure, yet the segmented form has remained the standard of the industry.
U.S. Pat. No. 1,388,424 issued to George in 1921 teaches the use of a unitary bit body having four conical cutters with axes nearly vertical, two being convergent and two being divergent. The cones and journals are shown integral, rotatively supported in bushings housed within the bit body, and fixed by threaded means. Unfortunately the cutting geometry of this design appears to be very non-aggressive.
Clarence Reed, a prolific inventor in the art, describes in U.S. Pat. No. 1,636,666 and more particularly in U.S. Pat. No. 1,692,793 a two cone rock bit of conventional cutting geometry featuring a one piece rock bit body. Individual journal shafts depend from vertical posts which are mechanically drawn into bores within the bit body by threaded means, after assembly of the rotary cutters.
Swift and Dalldorf were granted U.S. Pat. No. 1,726,049 on a unique rock bit having three cutters with vertical axes mounted in a straight line. The cylindrical cutters carried helical teeth which intermeshed to provide mutual cleaning and synchronous rotation. The cutters depend from a one piece bit body.
U.S. Pat. No. 2,061,657 by Howard, assigned to GLOBE OIL TOOLS COMPANY taught a design in which two cutters depend from a one piece bit body. Near vertical journal shafts demonstrate strong negative camber. The upper stator end of each journal member is drawn into a matching locking taper within the body and secured, in the production model, by a nut on a threaded extension of the journal member. The patent drawing, however, depicts use of a flat drive key with a locking taper. The cutting geometry was made effective by the use of the negative camber. The Howard patent was applied for in May of 1933, but before it came to issue in November of 1936, the well known three cone bit of current commerce, U.S. Pat. No. 1,983,316 by Scott et al, assigned to HUGHES TOOL COMPANY was issued, and has since pre-empted the marketplace.
An English inventor, Lanchester, in U.S. Pat. No. 2,648,526 teaches the use of a one piece bit body in a three cone rock bit. The independent journal shafts depend from cylindrical shanks which are threadingly drawn and secured into vertically converging bores within the bit body.
A novel cutting structure using three interleaving cutters with integral journal shafts having vertically converging axes rotatively supported by roller bearings within the one piece bit body, is described in U.S. Pat. No. 2,915,291 by Gulfelt.
The two latter designs seem never to have been successfully commercialized.
With the advent of ELECTRON BEAM WELDING, a number of patents have been issued directed to the use of this process in the production of rock bit designs having one-piece bit bodies U.S. Pat. Nos. 3,850,256 McQueen, 4,145,094 Vezirian, 4,158,973 Schumacher, 4,187,743 Thomas, and 4,256,194 Varel, are all illustrative of this trend. Although all of these efforts relied upon conventional prior art rotary cone cutting geometries, commercial use has not been seen. U.S. Pat. No. 4,209,124 by Baur, however, is directed to a fixture for electron beam welding a conventional segmented bit body together and is widely practiced.
U.S. Pat. No. 4,335,794 by Goodfellow shows a one-piece bit body with an open cylindrical "Pot" formed within the lower end. Cones are mounted on journals which depend from short "Legs" which are configured to fill the pot annularly, leaving a tapered bore at the center which is then filled with a tapered plug, in turn secured by a central bolt.
Generally, in a rotary cone rock bit, the centerlines of the individual rotary cones do not intersect the vertical centerline of the bit. By design, the cone centerline is displaced from the bit centerline by a certain small distance called the "offset". The offset is designed to "lead", meaning that the vertex of the cone advances, or "leads", about the bit center during normal drilling rotation, rather than retreating or "lagging". Offset introduces a small radial motion to the cutting tooth while it is in contact with the rock, increasing the cutting action of the tooth. Larger offsets are for use in relatively softer rock formations.
The individual "leg" of the conventional rotary cone rock bit is finished with radially extending flat surfaces which meet 120 degrees apart at the vertical bit centerline. These surfaces are mated to like surfaces on adjacent legs at assembly. The integral journal bearing shaft, formed by the lower end of the leg, is assembled with a rotary cone and then the legs are welded together along these vertically and radially eXtending flat faying surfaces to form the structure of the rock bit.
At assembly, a gage ring having an inside diameter equal to the diameter that the bit is intended to bore is placed around the cones. The cones are made to contact the inside diameter of the gage ring prior to welding the legs together, to insure that the bit is of correct size. Due to variability of parts and manufacturing tolerances, the fit to the gage ring may require adjusting. It is common practice to shift the legs with respect to one another along their faying surfaces to bring about that adjustment. This is done to the detriment of the built in offset dimension. Too much offset may cause premature failure of cutting teeth, while too little offset can reduce the rate of penetration of the rock bit.